1. Field of the Invention
The present invention relates to a safety device for use with a high-altitude working vehicle equipped with a vertically and horizontally swingable and extensible ladder assembly or a tower unit in which the movement of the ladder assembly or tower unit will be automatically stopped when such assembly or unit is subjected to an overload, thereby insuring the safety of the vehicle.
2. Description of the Prior Art
Generally, in a high-altitude working vehicle containing an extensible ladder assembly or tower unit, when the ladder assembly or tower unit is extended or contracted or vertically swung, the bending moment load on the ladder assembly or tower unit will change, and if an overload thereby occurs, the ladder assembly or tower unit may be broken or toppled over together with the vehicle body.
Heretofore, the detection of the bending moment load has been achieved by an arrangement comprising a beam extending along the base portion of a tower, the front end of the beam being fixed to the tower by means of bolts with the opposite or rear end of the beam being free, so that a bending load on the tower is converted into a displacement of the free end of the beam. Thus, when the tower is in a horizontal position, the tower will be bent in such a manner as to bring its front end nearest to the ground. Therefore, the front end of the beam is positioned below the level of the rear end. In other words, the free end of the beam will be upwardly displaced. This displacement is transmitted to a pointer through a segment gear, a rotatable arm and a pinion so that it may be read on an indicator plate. A switch is provided at a position corresponding to the allowable load limit and is adapted to be actuated by the rotatable arm to automatically stop the movement of the tower.
With this arrangement, however, flexure due to the self-weight of the beam will be found to influence the detected value, thus leading to an error. When the tower is in its vertical position, such flexure due to the self-weight of the beam becomes zero, since the beam will then be in its vertical position. As the beam is moved toward its horizontal position, the flexure will be found to increase. This error is very complex and can hardly be corrected.